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Year
A year is the of the moving in around the . Due to the Earth's , the course of a year sees the passing of the s, marked by changes in , the hours of , and consequently and . In and regions, generally four seasons are recognized: , , and . In seasonal and regions, the (rainy or ) season and the are generally recognized. A is an approximation of the Earth's orbital period in a given . The , presents the calendar year to be either a of 365 days or a of 366 days, as does the ; see below. The average year length across the complete leap cycle (400 years) of the Gregorian calendar is 365.2425 days. The ISO standard , Annex C, supports the symbol "a" (for Latin annus) to represent a year of either 365 or 366 days. In English, the abbreviations "y" and "yr" are used. In astronomy, the is a unit of time; it is defined as 365.25 days of exactly SI seconds each, totalling exactly seconds in the Julian astronomical year. "SI units" accessed February 18, 2010. (See Table 5 and section 5.15.) Reprinted from George A. Wilkins & IAU Commission 5, "The IAU Style Manual (1989)" (PDF file) in IAU Transactions Vol. XXB The word "year" is also used of periods loosely associated with but not identical to the calendar or astronomical year, such as the , the , or the , etc. Similarly, "year" can mean the orbital period of any : for example, a or a are examples of the time a planet takes to transit one complete orbit. The term can also be used in reference to any long period or cycle, such as the . , s.v. "year", entry 2.b.: "transf. Applied to a very long period or cycle (in chronology or mythology, or vaguely in poetic use)." Etymology ġēar ( ), ġēr continues *jǣran (*j ran). Cognates are Jahr, jār, ár and jer (Gothic e'' is always a long vowel), all from a noun ''*yeh₁rom "year, season". Cognates outside of Germanic are yārǝ "year", "year, season, period of time" (whence "hour"), jarŭ and hornus "of this year". Latin (a masculine noun; annum is the ; annī is genitive singular and nominative plural; annō the dative and ablative singular) is from a noun , which also yielded Gothic aþn "year" (only the dative plural aþnam is attested). Both *yeh₁-ro-'' and ''*h₂et-no-'' are based on verbal roots expressing movement, ''*h₁ey-'' and ''*h₂et-'' respectively, both meaning "to go" generally (compare ''éti "goes", atasi "thou goest, wanderest"). The Greek word for "year", , is cognate with Latin vetus "old", from the PIE word *wetos-'' "year", also preserved in this meaning in '' "yearling (calf)" and "year". Derived from Latin annus are a number of English words, such as , , , etc.; means "each year", means "in the year of the Lord". Civil year A calendar year is the time between two dates with the same name in a calendar. No astronomical year has an number of days or lunar months, so any calendar that follows an astronomical year must have a system of intercalation such as leap years. Financial and scientific calculations often use a 365-day calendar to simplify daily rates. In international calendars In the Julian calendar, the average (mean) length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every fourth year, or leap year, during which a leap day is intercalated into the month of February. The name "Leap Day" is applied to the added day. The Gregorian calendar attempts to cause the northward equinox to fall on or shortly before March 21 and hence it follows the northward equinox year, or tropical year.Ziggelaar, A. (1983). "The Papal Bull of 1582 Promulgating a Reform of the Calendar". In Coyne, Hoskin, Pedersen (eds), Gregorian Reform of the Calendar: Proceedings of the Vatican Conference to Commemorate its 400th Anniversary. Vatican City: Pontifical Academy of Sciences, Specolo Vaticano, p. 223 Because 97 out of 400 years are leap years, the mean length of the Gregorian calendar year is days; this is within one of the current length of the mean tropical year ( days) and even closer to the current March equinox year of days that it aims to match. It is estimated that by the year 4000 CE, the northward equinox will fall back by one day in the Gregorian calendar, not because of this difference, but due to the slowing of the Earth's rotation and the associated lengthening of the day. The Revised Julian calendar, as used in some es, currently does a better job than the Gregorian in synchronizing with the mean tropical year. As 218 out of every 900 years are leap years, the average (mean) length of this Julian year is days, which is closer to the length of the mean tropical year, days, than is the Gregorian mean year, 365.242 5 days. In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.Shields, Miriam Nancy. (1924). "The New Calendar of the Eastern Churches, Popular Astronomy, Vol. 32, p.407. Courtesy NASA Astrophysics Data System. A calendar era assigns a to each sequential year, using a reference point in the past as the beginning of the era. Worldwide, the most commonly used calendar era is referenced from the traditional—now believed incorrect—year of the birth of . Dates in this era are designated Anno Domini (Latin for in the year of the Lord), abbreviated AD, or the secular common era, abbreviated CE. The year before 1 AD, or 1 CE, is designated 1 Before Christ (BC), or 1 Before the Common Era (BCE), and the year before that is 2 BC/2 BCE, etc; hence, there was no /0 CE. When computations are done involving both years AD and years BC, it is common to use , in which 1 BC is designated 0, 2 BC is designated −1, and so on. Other eras are also used to enumerate the years in different cultural, religious or scientific contexts. In the Persian calendar The Persian calendar, in use in and , has its year begin at the midnight closest to the instant of the northward equinox as determined by astronomical computation (for the time zone of ), as opposed to using an algorithmic system of leap years. Fiscal year A fiscal year or financial year is a 12-month period used for calculating annual financial statements in businesses and other organizations. In many jurisdictions, regulations regarding accounting require such reports once per twelve months, but do not require that the twelve months constitute a calendar year. For example, in and the fiscal year runs from April 1; in the it runs from April 1 for purposes of corporation tax and government financial statements, but from April 6 for purposes of personal taxation and payment of state benefits; in it runs from July 1; while in the the fiscal year of the runs from October 1. Academic year An academic year is the annual period during which a student attends an . The academic year may be divided into s, such as semesters or quarters. The school year in many countries starts in August or September and ends in May, June or July. In Israel the academic year begins around October or November, aligned with the second month of the . Some schools in the UK and USA divide the academic year into three roughly equal-length terms (called trimesters or quarters in the USA), roughly coinciding with autumn, winter, and spring. At some, a shortened summer session, sometimes considered part of the regular academic year, is attended by students on a voluntary or elective basis. Other schools break the year into two main semesters, a first (typically August through December) and a second semester (January through May). Each of these main semesters may be split in half by mid-term exams, and each of the halves is referred to as a quarter (or term in some countries). There may also be a voluntary summer session and/or a short January session. Some other schools, including some in the United States, have four marking periods. Some schools in the United States, notably , may divide the year into five or more marking periods. Some state in defense of this that there is perhaps a between report frequency and academic achievement. There are typically 180 days of teaching each year in schools in the USA, excluding weekends and breaks, while there are 190 days for pupils in state schools in Canada, New Zealand and the United Kingdom, and 200 for pupils in Australia. In India the academic year normally starts from June 1 and ends on May 31. Though schools start closing from mid-March, the actual academic closure is on May 31 and in Nepal it starts from July 15. Schools and universities in Australia typically have academic years that roughly align with the calendar year (i.e., starting in February or March and ending in October to December), as the southern hemisphere experiences summer from December to February. In the International System of Quantities In the , the year (symbol, a'') is defined as either 365 days or 366 days. Astronomical years Julian year The Julian year, as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days. This is the normal meaning of the unit "year" (symbol "a" from the ''annus) used in various scientific contexts. The Julian century of days and the Julian millennium of days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many "real" years), for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a light-year. In the , the symbol, a'' (without subscript), always refers to the Julian year, ''aj, of exactly seconds. :365.25 days of seconds = 1 a = 1 aj = Ms The SI multiplier prefixes may be applied to it to form ka (kiloannus), Ma (megaannus), etc. Sidereal, tropical, and anomalistic years .}} Each of these three years can be loosely called an astronomical year. The {sidereal year is the time taken for the Earth to complete one revolution of its , as measured against a fixed frame of reference (such as the fixed stars, Latin sidera, singular sidus). Its average duration is mean solar days (365 d 6 h 9 min 9.76 s) (at the epoch = January 1, 2000, 12:00:00 ).International Earth Rotation and Reference System Service. (2010).IERS EOP PC Useful constants. Today the mean tropical year is defined as the period of time for the mean of the Sun to increase by 360 degrees.Richards, E. G. (2013). Calendars. In S. E. Urban & P. K. Seidelmann (Eds.), Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, CA: University Science Books. p. 586. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons; because of the biological and socio-economic importance of the seasons, the tropical year is the basis of most calendars. The modern definition of mean tropical year differs from the actual time between passages of, e.g., the northward equinox for several reasons explained below. Because of the Earth's , this year is about 20 minutes shorter than the sidereal year. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition. (= days of 86400 SI seconds) The anomalistic year is the time taken for the Earth to complete one revolution with respect to its . The orbit of the Earth is elliptical; the extreme points, called apsides, are the , where the Earth is closest to the Sun (January 3 in 2011), and the , where the Earth is farthest from the Sun (July 4 in 2011). The anomalistic year is usually defined as the time between perihelion passages. Its average duration is days (365 d 6 h 13 min 52.6 s) (at the epoch J2011.0). and the |location = Washington and Taunton |pages = A1, C2 }} Draconic year The draconic year, draconitic year, eclipse year, or ecliptic year is the time taken for the Sun (as seen from the Earth) to complete one revolution with respect to the same (a point where the Moon's orbit intersects the ecliptic). This period is associated with s: these occur only when both the Sun and the Moon are near these nodes; so eclipses occur within about a month of every half eclipse year. Hence there are two s every eclipse year. The average duration of the eclipse year is : days (346 d 14 h 52 min 54 s) (at the epoch J2000.0). This term is sometimes erroneously used for the draconic or nodal period of , that is the period of a complete revolution of the Moon's ascending node around the ecliptic: Julian years ( days; at the epoch J2000.0). Full moon cycle The is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the of the Moon's orbit. This period is associated with the apparent size of the , and also with the varying duration of the . The duration of one full moon cycle is: : days (411 days 18 hours 49 minutes 34 seconds) (at the epoch J2000.0). Lunar year The lunar year comprises twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days. use this for celebrating their s and for marking the start of the fasting month of Ramadan. A Muslim calendar year is based on the lunar cycle. Vague year The vague year, from annus vagus or wandering year, is an integral approximation to the year equaling 365 days, which wanders in relation to more exact years. Typically the vague year is divided into 12 schematic months of 30 days each plus 5 epagomenal days. The vague year was used in the calendars of , , and in among the and .Calendar Description and Coordination Maya World Studies Center It is still used by many Zoroastrian communities. Heliacal year A heliacal year is the interval between the s of a star. It differs from the sidereal year for stars away from the due mainly to the . Sothic year The Sothic year is the interval between heliacal risings of the star . It is currently less than the sidereal year and its duration is very close to the mean Julian year of 365.25 days. Gaussian year The Gaussian year is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by the . Such a planet would be slightly closer to the Sun than Earth's mean distance. Its length is: : days (365 d 6 h 9 min 56 s). Besselian year The Besselian year is a tropical year that starts when the (fictitious) mean Sun reaches an ecliptic longitude of 280°. This is currently on or close to January 1. It is named after the 19th-century German astronomer and mathematician . The following equation can be used to compute the current Besselian epoch (in years): |year=2008 |publisher = U.S. Government Printing Office and the U.K. Hydrographic Office |location = Washington and Taunton |page = B3 }} : B = 1900.0 + (Julian dateTT − ) / The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time. Variation in the length of the year and the day The exact length of an astronomical year changes over time.The Astronomical Almanac Online * The positions of the equinox and solstice points with respect to the apsides of Earth's orbit change: the equinoxes and solstices move westward relative to the stars because of , and the apsides move in the other direction because of the long-term effects of gravitational pull by the other planets. Since the speed of the Earth varies according to its position in its orbit as measured from its perihelion, Earth's speed when in a solstice or equinox point changes over time: if such a point moves toward perihelion, the interval between two passages decreases a little from year to year; if the point moves towards aphelion, that period increases a little from year to year. So a "tropical year" measured from one passage of the northward ("vernal") equinox to the next, differs from the one measured between passages of the southward ("autumnal") equinox. The average over the full orbit does not change because of this, so the length of the average tropical year does not change because of this second-order effect. * Each planet's movement is perturbed by the gravity of every other planet. This leads to short-term fluctuations in its speed, and therefore its period from year to year. Moreover, it causes long-term changes in its orbit, and therefore also long-term changes in these periods. * between the Earth and the Moon and Sun increases the length of the day and of the month (by transferring angular momentum from the rotation of the Earth to the revolution of the Moon); since the apparent mean solar day is the unit with which we measure the length of the year in civil life, the length of the year appears to decrease. The rotation rate of the Earth is also changed by factors such as and . Numerical value of year variation Mean year lengths in this section are calculated for 2000, and differences in year lengths, compared to 2000, are given for past and future years. In the tables a day is 86,400 SI seconds long. Values in tables agree closely for 2000, and depart by as much as 44 seconds for the years furthest in the past or future; the expressions are simpler than those recommended in the Astronomical Almanac for the Year 2011. Tabulates length of tropical year from −500 to 2000 at 500 year intervals using a formula by Laskar (1986); agrees closely with values in this section near 2000, departs by 6 seconds in −500. Summary An average Gregorian year is 365.2425 days (52.1775 weeks, hours, minutes or seconds). For this calendar, a common year is 365 days ( hours, minutes or seconds), and a leap year is 366 days ( hours, minutes or seconds). The 400-year cycle of the Gregorian calendar has days and hence exactly weeks. "Greater" astronomical years Equinoctial cycle The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years, and cannot be determined precisely as the speed is variable. Galactic year The Galactic year is the time it takes Earth's to revolve once around the . It comprises roughly 230 million Earth years. Seasonal year A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year. Symbols In the the symbol for the year as a is , taken from the Latin word annus. In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature, but also specifically in and , where "kyr}}, [[myr, byr" (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present. Symbol NIST SP811 and ISO 80000-3:2006 support the symbol a'' as the unit of time for a year. In English, the abbreviations ''y and yr are also used. The disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50http://aurora.regenstrief.org/~ucum/ucum.html#para-31 by using: :at = days for the mean tropical year; :aj = 365.25 days for the mean Julian year; :ag = days for the mean Gregorian year; where: :a'', without a qualifier = 1 aj; :and, ''ar for are, is a unit of area. The and the have jointly recommended defining the annus, with symbol a'', as the length of the tropical year in the year 2000: :''a = seconds (approximately ephemeris days) This differs from the above definition of 365.25 days by about 20 . The joint document says that definitions such as the Julian year "bear an inherent, pre-programmed obsolescence because of the variability of Earth’s orbital movement", but then proposes using the length of the tropical year as of 2000 AD (specified down to the millisecond), which of course suffers from the same problem. |year=2011 |volume=83 |pages=1159–1162 |doi=10.1351/PAC-REC-09-01-22 |issue=5}} |url=http://www.newscientist.com/article/dn20423-push-to-define-year-sparks-time-war.html |date=April 27, 2011 |accessdate=April 28, 2011}} (The tropical year oscillates with time by more than a minute.) The notation has proved controversial as it conflicts with an earlier convention among geoscientists to use a'' specifically for ''years ago, and y'' or ''yr for a one-year time period. SI prefix multipliers For the following, there are alternative forms which elide the consecutive vowels, such as kilannus, megannus, etc. The exponents and exponential notations are typically used for calculating and in displaying calculations, and for conserving space, as in tables of data. * ka (for kiloannus), is a one , or 103, years, or 1 E3 yr. The prefix multiplier "ka" is typically used in geology, paleontology, and for and periods where a non technique, i.e., dating, , , or analysis, is used as the primary dating method for age determination. If age is primarily determined by radiocarbon dating, then the age should be expressed in either radiocarbon or calendar (calibrated) years . * Ma (for megaannus), is a unit of time equal to one , or 106, years, or 1 E6 yr. "Ma" is commonly used in scientific disciplines such as geology, paleontology, and to signify very long time periods into the past or future. For example, the species was abundant approximately 66 Ma (66 million years) ago. The duration term "ago" may not always be indicated: if the quantity of a duration is specified while not explicitly mentioning a duration term, one can assume that "ago" is implied; the alternative unit "mya" does include "ago" explicitly. In astronomical applications, the year used is the Julian year of precisely 365.25 days. In geology and paleontology, the year is not so precise and varies depending on the author. * Ga (for gigaannus), is a unit of time equal to 109 years, or 1 E9 yr, one years (one years ). "Ga" is commonly used in scientific disciplines such as and geology to signify extremely long time periods in the past. For example, occurred approximately 4.54 Ga (4.54 billion years) ago. * Ta (for teraannus), is a unit of time equal to 1012 years, or 1 E12 yr, one years short scale (one billion years long scale). "Ta" is an extremely long unit of time, about 70 times as long as the . It is the same order of magnitude as the expected life span of a small . * Pa (for petaannus), is a unit of time equal to 1015 years, or 1 E15 yr, one short scale (one long scale). The half-life of the nuclide is about 8 "Pa". |volume = 76|page = 064603|year = 2007 |doi = 10.1103/PhysRevC.76.064603|bibcode = 2007PhRvC..76f4603B| issue = 6 }} This symbol coincides with that for the pascal without a multiplier prefix, though both are infrequently used and context will normally be sufficient to distinguish time from pressure values. * Ea (for exaannus), is a unit of time equal to 1018 years, or 1 E18 yr, one years short scale (one trillion years long scale). The half-life of is 1.8 "Ea". |volume = 67|page = 014310|year = 2003| arxiv = nucl-ex/0211013|doi = 10.1103/PhysRevC.67.014310|bibcode = 2003PhRvC..67a4310D }} Abbreviations yr and ya In , geology, and paleontology, the abbreviation yr for years and ya for years ago are sometimes used, combined with prefixes for thousand, million, or billion. They are not SI units, using y'' to abbreviate English year, but following ambiguous international recommendations, use either the standard English first letters as prefixes (t, m, and b) or es (k, M, and G) or variations on metric prefixes (k, m, g). These abbreviations include: Use of ''mya and bya is deprecated in modern geophysics, the recommended usage being Ma and Ga for dates Before Present, but "m.y." for the duration of epochs. This ad hoc distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America. Note that on graphs using ya units on the horizontal axis time flows from right to left, which may seem counter-intuitive . If the ya units are on the vertical axis, time flows from top to bottom which is probably easier to understand than conventional notation. References Notes Further reading * | year=1987 | isbn=0-87023-576-1 | oclc=15790499 }} * | year=2003 | isbn=0-19-860781-4 | oclc=265440481 }} Category:Units of time